By Trade Me's calculations, fridge trading had saved $4.2 million in environmental costs, he said.

DEREK BERWIN/GETTY IMAGES

Borthwick says fridges have all sorts of nasty gases in them that can harm the environment if they're not decommissioned properly.

"Fridges have all sorts of nasty gases in them that can harm the environment if they're not decommissioned properly … [and] it turns out that buying and selling fridges on Trade Me is bloody sustainable."

Trade Me has now nicknamed its fridge-selling members, and those buying them, "accidental eco warriors," he said.

"We reckon that most of them, like us, have no idea that they're being so eco-conscious. They just need to get rid of, or get themselves a fridge for the bach, garage or new flat …

"We know reselling and extending the useful life of items happens all day every day, but we haven't wrapped any maths around it for a long time."

"Buying and selling on Trade Me is pretty sustainable as a concept, but we didn't appreciate the scale of the impact that some of our categories could have on our environment.

"We figured the fridges category would be interesting, but we didn't think it would have saved nearly 33,000 tonnes of CO2."

Trade Me stumbled across its 'accidental eco-warriors' after deciding to look into ways the company was contributing to sustainable practices, Borthwick said.

It joined groups, such as the Sustainable Business Network (SBN), to better understand how the firm could help with issues like sustaining the environment, he said.

As a result, Borthwick attended some conferences and workshops with SBN members, when he was "struck by how active they thought Trade Me's second-hand marketplace was in benefitting the environment", he said.

"I discussed these assumptions with people around our business, and we thought we should validate them.

"[So] we enlisted the help of a sustainability expert Nic Bishop and decided to tackle one of the harder categories first and see what our environmental impact might be for that, with a view that we could roll out figures for other categories much faster."

Next in line could be furniture and other home and living items, he said.

"There's lots of reuse and upcycling opportunities with these things, and we can't wait to see the impact."

The company had not set out to be a sustainable company, though one of its key values was caring about the community which included being committed to making meaningful contributions where they could, he said.